This invention relates, in general, to semiconductor device fabrication and more particularly to a method of fabricating a heterojunction bipolar transistor that has relatively planar surface topology and reduced lateral dimensions.
Conventional methods of fabricating heterojunction bipolar transistors include forming collector, base and emitter layers on a substrate. Typically, the processes then form an emitter electrode followed by etching down to the base layer and forming a base electrode. Finally, an opening is etched to the collector layer so that a collector electrode may be formed. These conventional methods generally result in a heterojunction bipolar transistor having non-planar surface topology due to the large height difference between emitter electrode and the collector electrode.
Conventional single self-aligned heterojunction bipolar transistor fabrication processes generally align the base electrode only to the emitter electrode. This self-alignment results in a device configuration wherein the collector electrode is laterally distant from the base electrode. As a result, overall transistor size is sacrificed and the lateral dimensions of the transistor are larger than that obtainable using a process wherein the base electrode is automatically aligned to both the emitter and the collector electrodes.
An article by Tulley et al. entitled "A Fully Planar Heterojunction Bipolar Transistor", IEEE Electron Device Letters, Vol. EDL-7, No. 11, November 1986, page 615, discloses a planar, self-aligned heterojunction bipolar transistor fabrication process. This process utilizes a two-step epitaxial deposition with an intervening selective ion implant of bases. This epitaxial regrowth and selective ion implantation cause the disclosed process to be relatively complicated and difficult to employ.
In view of the above, it would be highly desirable to have a method of fabricating a heterojunction bipolar transistor that is relatively simple, results in a transistor having a relatively planar surface topology and reduced lateral dimensions and wherein the base electrode is self-aligned to both the collector and emitter electrodes.